1. Field of the Invention
The present invention relates to a method for manufacturing a nitride based single crystal substrate and a method for manufacturing a nitride based semiconductor device, and more particularly to a method for manufacturing a nitride based single crystal substrate and a method for manufacturing a nitride based semiconductor device, which prevent yield reduction due to cracks generated in a laser lift-off process.
2. Description of the Related Art
Generally, III-V group nitride based semiconductors emit light of a wide range from visible rays to ultraviolet rays. These nitride based semiconductors are widely used as optical elements for emitting ultraviolet rays and a bluish green color ray.
In order to apply a nitride based semiconductor to a light emitting device, a technique of growing a high-definition single crystal thin film of the nitride based semiconductor is required. Generally, the nitride based single crystal thin film is grown on a substrate made of a different material, such as a sapphire (Al2O3) substrate or a silicon carbide (SiC) substrate, by a hetero-epitaxy method using metal organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE).
Although the nitride based single crystal is grown on the sapphire substrate, since a difference of lattice constants between gallium nitride and sapphire is approximately 13% and a difference of thermal expansion coefficients between gallium nitride and sapphire is −34%, stress is generated from an interface between the sapphire substrate and the gallium nitride single crystal. Accordingly, in order to grow a nitride based semiconductor device directly on a nitride based single crystal substrate, a freestanding nitride based single crystal substrate is required.
As shown in FIG. 1, the freestanding nitride based single crystal substrate is obtained by growing a nitride based single crystal 15 on a preliminary substrate 11, such as a sapphire substrate, and separating the nitride based single crystal 15 from the preliminary substrate 11 using a laser lift-off process. The laser lift-off process is performed by decomposing the nitride based single crystal 15 at an interface region with the sapphire substrate 11 into metal by irradiating a laser beam thereonto, and separating the metal from the preliminary substrate 11 by melting. Here, a portion of the nitride based single crystal 15, onto which the laser beam is irradiated, is decomposed into metal, i.e., gallium (Ga), and nitrogen (½N2).
As disclosed in Korean Patent Registration No. 483049 (Title: method for manufacturing gallium nitride based light emitting device having vertical structure, Registrant: Samsung Electrics Co. Ltd., and Registration Date: Apr. 4, 2005), the laser lift-off process for separating the nitride based single crystal is used in the manufacture of a light emitting device having a vertical structure. That is, a method for manufacturing the nitride based light emitting device having a vertical structure is performed by bonding a permanent substrate to the upper surface of a nitride based light emitting structure formed on a preliminary substrate and separating the preliminary substrate and the nitride based light emitting structure from each other by the laser lift-off process.
However, the preliminary substrate is made of a material different from the nitride based light emitting structure, thus causing warpage and cracks (C) at the interface therebetween due to a difference of lattice constants and a difference of thermal expansion coefficients. Particularly, thermal stress due to the difference of thermal expansion coefficients severely occurs when the nitride based single crystal grown at a high temperature (900˜1,200° C. is cooled to the room temperature for the laser lift-off process.
In order to solve the above problem, as disclosed in Korean Patent Application No. 2005-0000265 (Title: method and apparatus for manufacturing gallium nitride based single crystal substrate, Applicant: Samsung Electrics Co. Ltd., and Application Date: Jan. 3, 2005), the laser lift-off process is carried out while a high-temperature condition is maintained. However, this method requires specially designed deposition equipment.
Accordingly, a method for more simply solving problems caused by stress between a nitride based single crystal or a nitride based light emitting structure and a preliminary substrate made of a material different from the nitride based single crystal or light emitting structure has been required.